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The fate of fer*liser phosphorus in pastures
A field study using isotopic tracers to label single superphosphate granules
Email: *[email protected] Phone: +61 8 8313 0394
Tim McLaren Ron Smernik Mike McLaughlin Richard Simpson Therese McBeath Chris Guppy Alan Richardson
Phosphorus in pasture systems
§ Managed grazing systems are the most geographically extensive land use worldwide, occupying ~ 25 % of the world’s land surface.
§ Phosphorus (P) is oNen limiOng in pastures and P ferOliser is essenOal for improved producOvity.
§ FerOliser P use efficiency of pastures is considered low and is thought to be as a result of soil ‘fixaOon’ processes.
Low
Fertiliser P (mostly inorganic P)
High
Scenario 1
Scenario 2
Plant P (mostly inorganic P)
Fertiliser P (mostly inorganic P)
Plant P (mostly inorganic P)
Soil P (inorganic/organic P)
Soil P (inorganic/organic P)
High High
High
Low
Experimental design 1
§ The aim of this study was to measure the fate of ferOliser P in clover pastures using 33P labelled single superphosphate (SSP).
§ Two field sites under permanent pasture (> 450 mm rainfall yr-‐1) - Ginninderra, near Canberra (Australian Capital Territory). - Kybybolite, near Naracoorte (South Australia).
§ Subterranean clover pasture (5 m × 5 m area)
§ PVC cores (15 cm Ø × 15 cm high) inserted to make in-‐tact cores.
§ Treatments included: - Control (no ferOliser) - Surface applied commercial SSP (~ 10 kg P ha-‐1 )
- Surface applied 33P labelled SSP granules (~ 10 kg P ha-‐1)
Experimental design 2
§ SSP was labelled with a 33P radionuclide using a “press and cut” technique.
§ Regular biomass cuts were taken (2 at Ginninderra and 4 at Naracoorte).
§ At the final cut, the remaining SSP granules were collected and the core soil separated into two layers (0 – 4 cm and 4 – 8 cm).
§ NaHCO3, NaOH-‐EDTA, and igniOon-‐H2SO4 extracOons of soil with P concentraOon (P-‐31) and radioacOvity (P-‐33) measured in the total and inorganic fracOons; organic P by difference.
Valida*on of 33P labelled SSP granules
33P labelled SSP granules Replicate 1 Replicate 2 Replicate 3 Replicate 4
Commercial SSP granules Replicate 1 Replicate 2 Replicate 3 Replicate 4
Degryse, F. and McLaughlin, M.J. (2014) ‘Phosphorus diffusion from ferOlizer: VisualizaOon, chemical measurements, and modeling’. Soil Science Society of America Journal 78: 832-‐842.
Dry maMer and clover P uptake
§ There was no significant difference between the control and 33P labelled SSP treatments for total dry maker and total P uptake at both sites.
Measurement Site Control 33P labelled SSP
Total dry maker (t DM ha-‐1) Ginninderra 6.7 (0.5) 7.3 (0.5) Naracoorte 8.0 (0.5) 6.9 (0.6)
Total P uptake (kg P ha-‐1) Naracoorte 28.4 (1.8) 28.1 (1.9)
Ginninderra 7.8 (0.7) 10.5 (0.8)
Sourcing fer*liser P
§ The proporOon of plant P derived from the P ferOliser was almost double at the Ginninderra site than at the Naracoorte site.
10
15
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30
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40
45
24/09/2013 14/11/2013
% of p
lant P derived
from
the
33P labe
lled SSP
Harvest date
Ginninderra
10 15 20 25 30 35 40 45
13/09/2013 1/10/2013 30/10/2013 5/11/2013
% of p
lant P derived
from
the
33P labe
lled SSP
Harvest date
Naracoorte
0
10
20
30
40
50
60
70
80
90
100
Naracoorte Ginninderra
% of applied P in components of the pasture system
Loca*on
Pasture tops
Remaining granule
Readily plant-‐available P (0 -‐ 4 cm layer)
Slowly plant-‐available P (0 -‐ 4 cm layer)
Unaccounted for P (roots??)
The fate of fer*liser P in pastures
Fer*liser P in soil frac*ons
§ Most ferOliser P recovered in NaHCO3 and NaOH-‐EDTA extracts was Pi and
in the 0 – 4 cm layer.
- Less than 5 % of ferOliser P had moved into the 4 – 8 cm layer.
§ Only a small amount (< 3 %) of the ferOliser P had accumulated as NaOH-‐
EDTA extractable Po in the 0 – 4 cm layer.
Conclusions
§ 34 and 40 % of the ferOliser P was recovered in the clover tops at the Ginninderra and Naracoorte field sites.
§ The proporOon of clover P uptake that was derived from the P ferOliser was higher at the Ginninderra site than at the Naracoorte site.
§ However, the soil was sOll the primary source of P for clover growth.
§ Approximately 30 % of the ferOliser P was found in soil surface (0 – 4 cm), which was largely Pi.
§ It is likely the remaining ferOliser P (~ 30 %) that was unaccounted for is contained within the root fracOon; we are currently tesOng this hypothesis.
The bigger picture
§ Phosphorus audits of long-‐term pasture soils oNen report high recoveries of ferOliser P (~ 90 %) in topsoil layers. - Hence, a low recovery by pastures is esOmated (~ 10 %).
§ Our results indicate that a relaOvely high proporOon of ferOliser P is uOlised by clover pastures, and when applied to pasture soils does not become rapidly unavailable to plants as previously thought.
§ Therefore, it is likely that a considerable proporOon of the ferOliser P used for pasture growth is actually recycled and returned to the soil surface.
§ It also appears the accumulaOon of Po in pasture soils is through long-‐term processes.
Acknowledgements
§ Financial support from Meat & Livestock Australia and the Australian Wool InnovaOon (Project B.PUE.0102).
§ Technical support from Ms Caroline Johnston, Mr Adam Stefanski and Ms Jane Carruthers.
§ John Cooper and Andrew Craig at the Kybybolite Research Centre (SARDI).